The specifications assume that in the intrinsically safe device, short-circuit faults could occur, which could possibly lead to a greater current drain, causing the device to lose its property as intrinsically safe.
To ensure that such an operating state cannot occur due to a malfunction of components in the device, the specifications demand a current limiting resistor in the power supply line. In the short-circuit case, up to 36 W of power dissipation can appear across this current limiting resistor. The specifications require that under no circumstances may this 36 W cause the temperature on the surface of the resistor to go over 135° C. To ensure that this temperature requirement can be maintained under unfavorable conditions, protective resistors with large surfaces and correspondingly large physical sizes are needed. Also the device housing must be able to dissipate the maximum power dissipation of 36 W in the interior without unacceptable heating.
With today's demands for device miniaturization, the large size of the protective resistor has proven to be a significant obstacle to further size reduction. Ultimately, the current limiting resistor determines the volume of the housing based on the required maximum surface temperature.
The same issue exists likewise for simple safety barriers. A safety barrier is a protective circuit that absolutely ensures that power-carrying lines that exit the safety barrier and run into the hazardous zone cannot carry a current or voltage that exceeds the permissible values for intrinsically safe power circuits.
The simplest type for such safety barriers uses an ohmic resistor in the series branch of the power path, which, if the safety barrier is to be arranged in the hazardous zone, must meet the same safety requirements for intrinsically safe devices as explained above. Thus, the safety barrier typically provides additional protective circuits in order to prevent over-voltages.